The short answer is that Thorium is just another fuel for fission reactors. It has some benefits and some drawbacks. For rockets, it's not a good choice. There are better fuels to use for fission if you want a fission rocket, and there are better choices than fission rockets for what we need today.

There's been a lot of hype recently in some quarters around thorium, but most people who know a lot about nuclear power aren't impressed by the hype. The hype ignores all the downsides to thorium and exaggerates the upsides.

what are the better[cheaper and efficient] fuel than thorium? [for a flying bus or an Iron man suite]to be able to fly without propellers and less noise/pollution

Thorium isn't going to power a flying bus or Iron Man suit in the foreseeable future. Neither is anything else.

Thorium goes in nuclear reactors that are very similar to all the nuclear reactors we currently use in electrical power plants, aircraft carriers, and submarines.

As to what is better/cheaper, the answer is uranium and plutonium. That's why we use uranium or plutonium in nearly all reactors in use today.

The people who design reactors aren't dumb. There's a reason we've been using uranium and plutonium for decades. It's not because people didn't know thorium existed. It's because they looked at the trade-offs and decided uranium and plutonium were the better choices.

You can start with the Wikipedia article for some advantages and disadvantages of thorium versus uranium and plutonium:

so,what are the better[cheaper and efficient] fuel for a flying bus or an Iron man suite?which could be the best fit for this specific purpose?

How do you define "flying bus"?

Depending on your definition, you might say a Bombardier CRJ700 jet is the closest thing we can get to a flying bus with current technology. It runs on kerosene. Or, if you define it as something that doesn't need a runway, then maybe the CH-47 Chinook helicopter is the closest thing with can get to a flying bus with current technology. It runs on kerosene too.

If you want something like what you see in Back to the Future, that looks like a road vehicle but can hover without any wings or rotors, you're going to be disappointed. We don't have the technology for that, or anything on the horizon that could give us that.

The Iron Man suit is also not anywhere close to being realistic with our current technology or any currently-forseeable technology.

the uber flying cars, human beings on big quad copters, and terrafugia kind of cars etcout of all i really like this design

a car without disturbing rotors

but it will be hard for this car engine to lift a bus [public transport]...so, probably it needs a different kind of engine

as the human population is exploding. it would be great if someone could comeup with an idea fora flying bus, or a flying train or a flying ship which can land/takeoff from the beaches/sea/lakes/rivers etc

imagine a swarm of flying cars overhead it would be better if there are few flying objects over my head

Don't forget that fission reactors unavoidably emit radiation that includes neutron radiation. That means you need a lot of heavy shielding, and it's not so good for your health to be sitting too close to the reactor, such as with the small vehicle you posted.

Iron Man's "arc reactor" is just a macguffin - a made-up device to advance a storyplot, which can do something radical/special and not really possible by today's technology.

But take a look at Lawrenceville Plasma Physics (LPP) and their Focus Fusion device. It's not super-huge (about the size of a small bathroom), and their plucky team keeps trying to improve it, bit by bit, in the hopes that it will one day provide gobs of energy.

as far as i know the primary advantages of Thorium are there are gobs and gobs and gobs and gobs of it and until it is ready to use it is basically harmless. so if a freight car of it exploded and fell over the U.S. it would just be a falling object problem and not an environmental catastrophe that needed billions for clean up, medical treatment and relocation monies.

It is my understanding that Thorium reactors must first convert a working amount of harmless Thorium to a highly radioactive relatively short lived isotope of Uranium. So without knowledge of the stuff necessary to do that conversion (probably some sort of reactor in and of itself plus ancillary systems ) it would seem to be simple to only convert the Thorium as needed moment by moment to power the main reactor. But if you had to have a reactor to convert the Thorium you're back to the original problem anyway; flying dangerous nuclear material over the Earth's surface with a finite risk of horrible accidents.

as far as i know the primary advantages of Thorium are there are gobs and gobs and gobs and gobs of it and until it is ready to use it is basically harmless. so if a freight car of it exploded and fell over the U.S. it would just be a falling object problem and not an environmental catastrophe that needed billions for clean up, medical treatment and relocation monies.

It is my understanding that Thorium reactors must first convert a working amount of harmless Thorium to a highly radioactive relatively short lived isotope of Uranium. So without knowledge of the stuff necessary to do that conversion (probably some sort of reactor in and of itself plus ancillary systems ) it would seem to be simple to only convert the Thorium as needed moment by moment to power the main reactor. But if you had to have a reactor to convert the Thorium you're back to the original problem anyway; flying dangerous nuclear material over the Earth's surface with a finite risk of horrible accidents.

You need to compare apples to apples. Here you're implicitly comparing thorium to uranium-235 (the radioactive kind of uranium, and the kind used in most nuclear reactors).

But thorium can only be used as a fuel in a breeder reactor. If you're going to have a breeder reactor, you can use uranium-238. Uranium-238, like thorium isn't radioactive and can't itself be used as a nuclear reactor fuel. But it can be hit with neutrons to make a radioactive isotope that can be used as nuclear reactor fuel, just like thorium.

And, while there isn't as much uranium-238 on Earth as there is thorium, there's still an incredibly huge amount of it -- enough to power our civilization for thousands of years. And if a freight car of uranium-238 exploded and fell over, it would be just as harmless as if a freight car of thorium did the same.

Thorium doesn't have some advantages, but those qualities you listed aren't advantages over uranium-238 used in a breeder reactor.

it is my understanding based on dim memory of years old articles that I read way in the past; that the type of uranium involved is neither the common barely radioactive natural uranium nor the type commonly used in reactors or bombs. but a different even more highly radioactive isotope.

it is my understanding based on dim memory of years old articles that I read way in the past; that the type of uranium involved is neither the common barely radioactive natural uranium nor the type commonly used in reactors or bombs. but a different even more highly radioactive isotope.

I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs. Again i don't retain all the details of stuff i read over periods of years and i would be lucky if i got all that right from my poor memory.

as far as i know the primary advantages of Thorium are there are gobs and gobs and gobs and gobs of it and until it is ready to use it is basically harmless. so if a freight car of it exploded and fell over the U.S. it would just be a falling object problem and not an environmental catastrophe that needed billions for clean up, medical treatment and relocation monies.

Neither is uranium. If "freight car of U exploded and fell over the U.S", no cleanup would be necessary.

If that's pure U235, authorities would be recovering those chunks of it which are large-ish (since it's a bomb material). If pulverized to tiny particles and distributed over US, it's not recoverable for someone trying to build a bomb, and not hazardous (there are thousands of tons of it in exposed granite rocks anyway).

I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs. Again i don't retain all the details of stuff i read over periods of years and i would be lucky if i got all that right from my poor memory.

You are bringing up an entirely separate advantage of Thorium, the nuclear weapons proliferation issue. That doesn't justify your saying "you would be right about that except".

No. I was right in what I said, with no "except". I never said there wasn't a weapon proliferation issue. I even said Thorium had other advantages and disadvantages. My point was just that the two issues you brought up originally as advantages of thorium aren't really advantages over uranium breeder reactors.

I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs. Again i don't retain all the details of stuff i read over periods of years and i would be lucky if i got all that right from my poor memory.

You are bringing up an entirely separate advantage of Thorium, the nuclear weapons proliferation issue. That doesn't justify your saying "you would be right about that except".

No. I was right in what I said, with no "except". I never said there wasn't a weapon proliferation issue. I even said Thorium had other advantages and disadvantages. My point was just that the two issues you brought up originally as advantages of thorium aren't really advantages over uranium breeder reactors.

I did not mean to distract you with proliferation. i just didn't want to multipost so i put that in there.

The meat of it (provided I am correct and not misremembering) is that the Thorium can be converted to U-233 in what amounts to an almost instant manner as needed. I do not think that is the case for normal uranium breeder cycles. if that memory is correct then Thorium could be used in rocket applications with more safety and ease than U238-U235 because the U 235 would need to be made in advance and in the final quantities needed over the between refurbishment lifetime of the craft. The thorium thing would only ever have a hopper full of harmless thorium and a the moment to moment needed amount of U 233 in the reactor.

I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs.

That's wrong - there would be more than enough U233 to build a bomb. Reactors have hundreds of tons of fuel in them. 1% burnup of thorium in such a reactor would yield some ~1 ton of U233 - enough for a hundred nukes.

The problem (for bomb makers, that is) is that thorium reactors also generate U232, which has very undesirable radiological properties, contaminating that U233.

The meat of it (provided I am correct and not misremembering) is that the Thorium can be converted to U-233 in what amounts to an almost instant manner as needed. I do not think that is the case for normal uranium breeder cycles.

I don't think you are remembering it right. IIRC both processes are similar:

Th232+n -> Th233 -> Pa233 -> U233U238+n -> U239 -> Np239 -> Pu239

Both are beta decay chains with several days half-lives in the second decay.

The meat of it (provided I am correct and not misremembering) is that the Thorium can be converted to U-233 in what amounts to an almost instant manner as needed. I do not think that is the case for normal uranium breeder cycles.

I don't think you are remembering it right. IIRC both processes are similar:

Th232+n -> Th233 -> Pa233 -> U233U238+n -> U239 -> Np239 -> Pu239

Both are beta decay chains with several days half-lives in the second decay.

damn. I was nearly certain i had read that part of it. i thought there was an article with the conversion being done in flight. but i guess i was wrong. not that i recall the detail of how rapidly the fuel was used up either.

I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs.

That's wrong - there would be more than enough U233 to build a bomb. Reactors have hundreds of tons of fuel in them. 1% burnup of thorium in such a reactor would yield some ~1 ton of U233 - enough for a hundred nukes.

The problem (for bomb makers, that is) is that thorium reactors also generate U232, which has very undesirable radiological properties, contaminating that U233.

U233 is a strong gamma-radiator, which makes it too deadly for anyone to isolate into a bomb and carry around.

Likewise, you wouldn't want to be enriching it aboard a spacecraft either, without a lot of shielding and distance for safety.

Bur Carlos Rubbia's idea for an Accelerator-Driven Reactor system might be useful for nuclear propulsion one day, if the mass requirements can be kept down. Using fuels like Thorium or U238 which are merely fertile not fissile would at least avoid any danger of runaway chain-reaction meltdown. As soon as the accelerator beam is cut, the nuclear reaction is immediately quenched or shut off.